Traditionally, dimming of hot cathode fluorescent lamps is accomplished by controlling the operating frequency of a series resonant inverter that drives all the lamps in series. A closed loop control circuit regulates the lamp current or power to adjust the lumen output of the lamp to provide dimming.
In order to provide a satisfactory life of the lamp, a cathode voltage is provided to the lamp cathodes with increasing value as the lamp is dimmed. This applied cathode voltage has the effect of heating the cathode in such a way as to reduce the sputtering effect of the lamp at lower operating currents when operated in a dimmed mode. The cathode voltage continuously supplies the cathode heating, although at an increased voltage, as the lamp is dimmed.
The dimming system and method described heretofore has some disadvantages. First, a series lamp configuration results in an increase in maintenance costs relative to a parallel lamp configuration. All lamps in a series configuration will fail if one lamp fails. This failure mode necessitates service calls every time one lamp fails. Secondly, a continuously supplied voltage to the cathodes, even when the lamp is providing 100% lumen output, is an inefficient technique for dimming. The cathodes dissipate up to 3 watts or 10% of the system power for each lamp without producing any visible light.
This disclosure provides a ballast circuit and method of dimming lamps that overcomes some of the disadvantages associated with a continuously supplied cathode voltage lighting system. In addition, this disclosure also demonstrates a method for parallel lamp dimming.